The invention relates to an electro-hydraulic control circuit for an electronically controlled powershift transmission.
Microprocessor-based electronic controllers for powershift transmissions make possible features which are not otherwise practical. However, it is difficult to achieve quality shifts with an electronically controlled transmission. Because coordination of multiple elements is difficult, electronically controlled powershift transmissions have required substantial, time consuming and expensive fine tuning in order to achieve satisfactory overall shift quality. This is because the microprocessor or hydraulic valving which controls the transmission does not receive sufficient information (feedback) about what is actually going on inside the transmission.
In an electronic microprocessor-based control system for a powershift transmission, the microprocessor generates time-based electrical signals with extreme accuracy and repeatability, but the hydraulically operated transmission components respond directly only to hydraulic pressures generated by valves which are controlled by the electronics. The electrical signals are adjusted to generate, after some series of events, the correct pressure outputs for one shift condition, but any change in temperature, air/oil mixtures, seal drag, valve gain, engine speed, or dozens of other variables will modify the hydraulic pressures which are generated, perhaps resulting in a rough shift. Control systems are available wherein the electronic controller is supplied with electronic signals corresponding to such variables. However, such systems have not been able to provide quality shifts under all circumstances. Using electronic feedback for parameters, such as clutch piston position, actual pressure on the piston, actual force on clutch discs, to solve this problem is impractical because of cost and complexity. Accordingly, it would be desirable to have a control system which achieves satisfactory shifts and which does not require extensive electronic variable sensing and which does not require electronic sensing of such feedback parameters.